docstr: Receives a tensor from an (optional) source src.
docstr_tok: ["Receives","a","tensor","from","an","(","optional",")","source","src","."]
code: def recv(self, tensor, src=None):
      """Receives a tensor from an (optional) source src."""
      rank, result = self.mailbox[self.rank].get()
      if src is not None and rank != src:
          raise NotImplementedError(
              "Can't receive messages out of order yet"
          )
      return result
  
imports: import logging
  import threading
  from operator import itemgetter
  from queue import Queue
  import torch
  from torch.distributed import ReduceOp
  from .communicator import Communicator
label_desc: Use image node_id 6 for calling the InProcessCommunicator obj's underlying member method code with example usage: obj.recv(tensor, src) and returns: result